1. FIELD OF THE INVENTION
This invention relates generally to the field of semiconductor manufacturing, and, more particularly, to a method and apparatus for controlling byproduct induced defect density in semiconductor wafers.
2. DESCRIPTION OF THE RELATED ART
In the manufacture of semiconductor devices, wafers, such as silicon wafers, are subjected to a number of processing steps. The processing steps include depositing or forming layers, patterning the layers, and removing portions of the layers to define features on the wafer. One such process step is the formation of a layer by chemical vapor deposition, where reactive gases are introduced into a vessel containing the semiconductor wafers. The reactive gases facilitate a chemical reaction that causes a layer to form on the wafers.
Certain chemical reactions produce byproducts in addition to the desired material forming the layer. Some of these byproducts vaporize at the elevated processing temperatures at which the deposition process operates, and absorb into the forming layer, causing small particle defects in the film. As subsequent lots of wafers are processed, the concentration of the byproducts increases, causing a corresponding upward trend in the defect level on the wafers.
This increasing defect level may be monitored by known statistical process control techniques. When the defect level reaches a certain level, the processing tool is disassembled and cleaned to remove the byproduct materials, thus maintaining the defect density at acceptable levels.
An exemplary processing tool susceptible to byproduct production is a low pressure chemical vapor deposition (LPCVD) system used to deposit a silicon nitride Si.sub.3 N.sub.4 layer. Silicon nitride layers are commonly used as passivation, masking, or insulating layers. In the LPCVD system, dichlorosilane (SiH.sub.2 Cl.sub.2) is combined with ammonia (NH.sub.4) to facilitate the chemical reaction that results in the deposition of the silicon nitride layer. An ammonium chloride (NH.sub.4 Cl) byproduct forms and builds up on the internal surfaces of the processing chamber. As nitride layers are formed on subsequent lots of wafers, the ammonium chloride deposits as particle defects within the silicon nitride layers. When the defect count reaches a predetermined level, the LPCVD tool is disassembled and cleaned.
Typically, the disassembly, cleaning, and re-assembly process takes at least 24 hours. The maintenance procedure is generally conducted after between 20 and 30 processing runs. The significant down time required to complete the cleaning procedure affects the production efficiency of the tool and the overall facility.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.